Article pubs.acs.org/Langmuir
An Abrasion-Resistant and Broadband Antireflective Silica Coating by Block Copolymer Assisted Sol−Gel Method Liping Zou,† Xiaoguang Li,† Qinghua Zhang,‡ and Jun Shen*,† †
Shanghai Key Laboratory of Special Artificial Microstructure and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China ‡ Chengdu Fine Optical Engineering Research Centre, Chengdu 610041, China ABSTRACT: A double-layer broadband antireflective (AR) coating was prepared on glass substrate via sol−gel process using two kinds of acid-catalyzed TEOS-derived silica sols. The relative dense layer with a porosity of ∼10% was obtained from an as-prepared sol, while the porous layer with a porosity of ∼55% was from a modified one with block copolymer (BCP) Pluronic F127 as template which results in abundant ordered mesopores. The two layers give rise to a reasonable refractive index gradient from air to the substrate and thus high transmittance in a wide wavelength range, and both of them have the same tough skeleton despite different porosity, for which each single-layer and the double-layer coatings all behaved well in the mechanical property tests. The high transmittance and the strong ability of resisting abrasion make this coating promising for applications in some harsh conditions. In addition, the preparation is simple, low-cost, time-saving, and flexible for realizing the optical property.
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INTRODUCTION Antireflective (AR) coatings have been widely used in many areas such as laser systems, solar cell systems, and displaying systems to increase the availability of light.1−8 The AR coating can be prepared by both physical and chemical approaches. The former is called physical vapor deposition (PVD) by which the optical parameters of coating can be precisely controlled and multilayers can be easily realized; the limitations and drawbacks mainly represent in the application of large or complex substrates, the time-consuming deposition process, the limited low-index materials selection (e.g., MgF2), and the high cost for some industrial productions. Nowadays, to address these issues, wet chemical methods have been widely studied such as layerby-layer (LbL) assembly, sol−gel process, and so on, and nanoparticle AR coatings are very representative in the products.9−16 Relatively, the sol−gel AR coatings were studied earlier and have been well applied for many years, especially in the high power laser system. The sol−gel technique features good flexibility in the porosity control for coating preparation, which means the availability of desired refractive index for antireflection.13−16 Silica is the most universally used material in the sol−gel preparation of AR coatings. There are two kinds of silica sols distinguished by acid and base catalysis. The coating from base-catalyzed silica sol (base-SiO2) is accumulated by cross-linked nanoparticles, having a high porosity and thereby a low reflective index (∼1.20), which can actualize approximately zero reflection but is sometimes limited in application for its weak mechanical property. In contrast, the coating from the acid-catalyzed silica sol (acid-SiO2) are composed of dense fiber chains resulting in low porosity and © 2014 American Chemical Society
high refractive index (∼1.40), which can decrease the reflectance from ∼8% to ∼4% for glass substrate. Although it is not perfect antireflection, the excellent mechanical property makes this kind of AR coating capable for the application in some harsh environments. Besides, the acid-SiO2 sol is very stable compared with the base-SiO2 sol that would evolve into wet gel with time. There is a way to further decrease the reflectance and maintain the tough skeleton at the same time for the acid-SiO2 coating, by doping the sol with block copolymers that form micelles via self-assemble in the coating phase, and followed by burning them out to create pores.17−25 In our work, Pluronic F127a triblock copolymerwas used as pore former considering its compatibility with silica systems,22−25 by which we obtained abrasion-resistant AR coating (F127-SiO2) with transmittance peak being as high as 99.9%. The single-layer uniform AR coating is accompanied by a Vshaped reflection spectrum that means the transmittance decreases fast with the wavelength deviating from the peak position. However, in some situations such as the displaying system and the solar cell system, a wide wavelength range of light is required, and thus broadband AR coatings should be employed.4,5,15,16,20,26 Broadband AR coatings usually consist of multilayers with different refractive indices. Because of the prevailing infiltration between adjacent layers and the difficulty to precisely control the layer thickness, the double-layer coating Received: June 20, 2014 Revised: August 3, 2014 Published: August 12, 2014 10481
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nanoindentation measurements with TriboIndenter in situ nanomechanial test system (TriboIndenter Hysitron). The films were scratched by a conical diamond tip with 2 μm diameter, scanning over a 10 μm track and indented by a Berkovich-type pyramidal diamond tip. Both nanoscratch and nanoindentation tests were performed at room temperature with a humidity of ∼50%.
system is the most popular choice for sol−gel broadband AR coating, in which λ/4−λ/2 (λ is the center wavelength) and λ/ 4−λ/4 film designs are usually adopted. In terms of optical property, the λ/4−λ/4 design that refers to two layers forming refractive index gradient and having the same optical thickness of λ/4 is better than the other one. However, the outer layer in this case is always porous material with poor mechanical strength which limits its application. To increase the strength, a rigorous process such as high temperature (above 600 °C) treatment27 or ammonia treatment (above 200 °C)16 is indispensable which is power-wasting or inconvenient. For this, we prepared a λ/4−λ/4 broadband AR coating with the outer layer being the F127−SiO2 film and the inner layer being the acid−SiO2 film that are introduced above. Owing to the intrinsic tough skeleton, the coating presented excellent mechanical strength which was proved by abrasion test, nanoscratch, and nanoindentation measurements. The two layers were deposited both by the dip-coating method, and the calcination treatment process was employed only after the second deposition. This work provides a low-cost and simple way to obtain durable broadband AR coating for applications requiring abrasion resistance.
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RESULTS AND DISCUSSION Single-Layer Coating with High Porosity. The mesopores were produced by burning out the micelle template that formed via the EISA (evaporation-induced self-assembly)28,29 process, in which the solution of silica and template agent F127 (a surfactant composed of a hydrophobic chain in the middle and hydrophilic chains at both ends) was casted to a substrate, followed by alcohol evaporation that resulted in the concentration increase of F127 and therefore its self-assembling into micelles that acted as the structure template. The TG-DSC test of the F127−SiO2 gel was carried out to analyze the coating composition (Figure 1a). The TG curve
EXPERIMENTAL SECTION
Materials. Tetraethyl orthosilicate (TEOS), ethanol (EtOH), and concentrated hydrochloric acid (HCl) of reagent grade were purchased from Sinopharm Chemical Reagent Co., Ltd. Pluronic F127 (EO106PO70EO106, Mw = 12 800) was purchased from Aldrich. All of the reagents were used as received without further purification. Preparation of the Acid−SiO2 Sol. Tetraethyl orthosilicate (TEOS) was mixed with ethanol (EtOH), distilled water (H2O), and concentrated hydrochloric acid (HCl). The mole ratio was TEOS:EtOH:H2O:HCl = 1:38:2.24:0.21. This clear and acidic solution was ready for use after 2 h ripening at room temperature. Preparation of the F127−SiO2 Sol. Tetraethyl orthosilicate (TEOS) was mixed with half of the ethanol (EtOH). Distilled water (H2O) and concentrated hydrochloric acid (HCl) were added into it after 10 min stirring, and the mixture was noted as solution A. F127 was dissolved into the other half of the ethanol at 40 °C, and the mixture was noted as solution B which was then dropwise added with solution A with stirring for 2 h. The resulting solution is transparent in which the mole ratio of reagents is TEOS:ETOH:H2O:HCl:F127 = 1:38:4.48:0.46:9.95 × 10−3. The solution was ready for use after 2 days aging in a sealed glass container. Preparation of the Broadband AR Coating. A clean glass substrate was successively coated with the acid−SiO2 and the F127− SiO2 sols by the dip-coating method, with the withdraw rates of 2.5− 4.4 and 0.5−0.8 mm/s, respectively. Just 2 min waiting was needed for the first layer drying in the air. The two-layer coated glass was dried at room temperature for 10 min and was then moved into a muffle oven, annealing at 280 °C for 5 min to remove the polymer template. Characterization. The transmittances of the coated and uncoated glasses were measured by a UV−vis−NIR spectrophotometer (Jasco570), and the refractive indices of the films were determined by modeling of the transmittance spectra using Film Wizard32 software. The simulation was carried out with the optical coating design program Essential Macleod. The surface morphology of the coating was characterized by an atomic force microscope (AFM, XE-100 Park) in the noncontact mode and a field-emission scanning electron microscope (FESEM, Philips-XL-30FEG). The structure of the film was studied with transmission electron microscopy (TEM, JEOL1230) and small-angle X-ray diffraction (SAXD, Bruker D8 Discover X-ray diffractometer). The composition of the coating and the decomposition details were studied with Fourier transform infrared spectroscopy (FTIR, Bruker, TENSOR 27) and TG-DSC (NETZSCH STA499C). The mechanical properties of the films were tested by a rubbing experiment and by nanoscratch and
Figure 1. TG-DSC curves of the F127−SiO2 gel (a) and the FTIR spectra of the F127−SiO2 coating (b).
reveals the total weight loss of ∼58% in the temperature range from 150 to 300 °C. Correspondingly, the exothermic peaks of the DSC curve at 180 and 238 °C locate in the region of main weight loss, which can be ascribed to the liberation of condensed water or hydroxide and the decomposition of F127 polymer, respectively. The results indicate that the F127 template could be mostly removed by annealing up to 280 °C. The removal of F127 in the silica coating is further confirmed by the FTIR spectra that are shown in Figure 1b. After annealing at 280 °C for 5 min, the stretching vibrations of −CH3 and −CH2 distributed at 2885 and 2968 cm−1 disappear. Likewise, the peaks at 842 and 1112 cm−1 assigned to the stretching vibration of C−O−C and the peaks at 1296 and 1360 cm−1 assigned to the bending vibration of C−H30,31 all disappear. These results imply that the F127 template has been removed by annealing at 280 °C, which is consistent with the TG-DSC analysis. The addition and removal of F127 resulted in the formation of mesoporous silica with ordered channels as shown in Figure 2. The TEM images indicate that the film structure is highly ordered with a uniform pore size of ∼9 nm (Figure 2a,b) and is presented as hexagonally packed cylinders with a period of ∼6 nm (Figure 2c). The SAXD curve of the film shows sharp diffraction peaks (Figure 2d), meaning the existence of an ordered mesopore structure, coinciding with the TEM results. The AFM images and transmittance spectra of the acid−SiO2 and the F127−SiO2 coated glasses are displayed in Figure 3. As revealed, the acid−SiO2 coating gives rise to ∼4% increase of the transmittance compared with the bare glass and features a flat surface with the rms (root-mean-square) roughness as low as that of the glass (∼1.0 nm). In contrast, the F127−SiO2 10482
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Double-Layer Coating with Broadband AR Transmittance. Broadband AR coatings derived from chemical methods have been studied for many years. Yoldas and Partlow35 prepared one via etching a porous silica coating with hydrofluoric acid. Yamaguchi36 obtained such coatings via etching sol−gel alumina films in boiling water. This kind of etching method was aimed to simulate the structure of moth eyes to get continuous refractive index to realize the broadband antireflection. In contrast to etching, the two-layer coating system is easier to control with better operational security. As introduced, the 1/4λ−1/4λ broadband AR coating designed in this work is quite simple to prepare without dangerous operation such as the use of hydrofluoric acid. As demonstrated in Figure 4, the
Figure 2. Front (a, b) and side (c) TEM images and the SAXD curve (d) of the F127−SiO2 film calcinated at 280 °C.
Figure 4. Transmittance spectrum and the schematic illustration of the glass with two-layer broadband AR coating.
coating is made up of two layers: the inner dense layer and the outer ordered porous layer. Each layer thickness was designed as a quarter of the center wavelength, which was carried out mainly by adjusting the withdraw speed. The 1/4λ−1/4λ design requires reasonable refractive index gradient for desirable optical performance. In our experiment, the refractive indices of the dense and the porous layers are in the ranges of 1.41−1.45 and 1.21−1.28, respectively, relating to the aging time of the sol and the calcination temperature. From these we can reckon the porosities according to Lorentz−Lorenz relationship:37
Figure 3. Transmittance spectra of the acid−SiO2 (a) and the F127− SiO2 coated glasses (b). The insets are AFM images of the two coatings separately.
coating presents a surface morphology of massive sags and domes (Figure 3b) with the rms roughness of ∼1.9 nm. The slight increase of the roughness would not do any harm because it is still far below the concerned wavelength. The transmittance at center wavelength is above 99.9%, 8.2% higher than that of the bare substrate (with a refractive index of 1.52), indicating that the refractive index of the coating is close to the optimum value (1.23) for antireflection. The mechanical property evaluation of the coating is important, and there are a few popular options. For example, measurements with sclerometers and adhesive tape are among the most common methods. Sand abrasion tests are sometimes performed to simulate the outdoor application.27 Recently, nanostratch and nanoindentation tests are found very useful to evaluate the mechanical property quantitively32,33 and were adopted in this study with the results discussed in later sections. Herein, we employed another method that is suitable for testing the abrasion resistance of AR coating suggested by Floch et al.,34 i.e., rubbing the coating with alcohol cotton balls under ∼2 N of the pressure and then comparing the transmission spectra before and after rubbing. This test method has been employed by many researchers and is considered very suitable for comparison purposes.15,16,34 In our experiment, no obvious damage could be found on the F127−SiO2 coating by visual inspection after 100 times rubbing, and the transmission spectra before and after rubbing are nearly identical to each other as shown in Figure 3b.
(n f 2 − 1)/(n f 2 + 2) = (1 − Vf )(ns 2 − 1)/(ns 2 + 2) (1)
where nf and ns are the refractive indices of the porous film and the solid skeleton (1.46), respectively, and Vf is the porosity, namely the volume fraction of the pores. Hence, porosities of the dense and the porous layers are ∼10% and ∼55%, respectively. In these ranges, the higher the index of the dense layer and the lower that of the porous layer, the better the optical performance, simulated by the software of Essential Macleod based on the film optics theory. For the sample corresponding to Figure 4, the average transmittance is more than 99% over a wide range from 700 to 1240 nm with the center wavelength of ∼950 nm, and the refractive indices of the two layers are 1.45 and 1.25. Although the peak transmittance is lower than that of the single-layer (F127−SiO2) sample, the spectrum is much flatter which could satisfy some special demands. The range of broadband antireflection can be easily adjusted to meet different requirements. Figure 5 displays a sample with high transmittance in the visible region which is required in many cases such as display windows and solar cell glasses. It can be seen that the coated part presents a significantly improved 10483
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and the diamond, which increases with the insertion depth. The friction coefficient is defined as the ratio of the friction and the applied normal load, and a small value indicates a shallow insertion depth, which means good scratch resistance. As shown in Figure 6, the acid−SiO2 coating presents best on the
Figure 5. Transmittance spectrum with a slight deviation from 1/4λ− 1/4λ design and the photograph (the inset).
transparency compared with the bare part. The two layers, with refractive indices of 1.415 and 1.217, respectively, improve the transmittance to 99.7% at the peak point and above 99.0% over a wide range from 400 to 950 nm. The layer thicknesses are 86 and 113 nm, 0.24 and 0.27 times of the center wavelength (510 nm), respectively, deviating from the 1/4λ−1/4λ design. Nevertheless, desirable optical performance can also be obtained, and this allowable error in the thickness control makes the preparation method more applicable in some sense. The rubbing test introduced above was employed in a comparative experiment with the results given in Table 1. For
Figure 6. Variation of the friction coefficient as a function of applied load for the acid−SiO2, F127−SiO2 and double-layer coatings. The sudden drop in the initial load region was caused by the settling down of the indenter head.
whole the friction coefficient of which increases from 0.12 to 0.21 between 200 and 1400 μN. The F127−SiO2 coating behaves similar in the range of 200−900 μN but rises faster in the following section which means worse scratch resistance compared with the acid−SiO2 coating. The friction coefficient of the double-layer coating increases from 0.14 to 0.28 between 200 and 1400 μN and keeps highest values over the whole range. According to the results, all the three coatings present good scratch resistance, and the double-layer coating is the weakest for any given applied load, which is in coincidence with the rubbing test. The coating hardness and reduced modulus were determined using the nanoindentation method. Figure 7 shows the load−
Table 1. Optical and Mechanical Properties of Different Single-Layer and Double-Layer SiO2 AR Coatings transmittance decrease after rubbing (%)
coating category
temp (°C)
peak transmittance (%)
F127−SiO2 coating
200
97.10
240 280 200 280
acid−SiO2 coating double-layer coating
100 times
300 times
500 times
99.79 99.93 95.30
1.6 ∼0 ∼0
0.12 0.11
0.23 0.21
99.70
∼0
0.15
0.27
the single-layer of F127−SiO2 coating, different treatment temperatures were attempted, which lead to different transmittances and mechanical properties. The transmittance increased along with the temperature increase, indicating the gradual removal of F127. The transmittance is as high as 99.79% for 250 °C treatment, but the abrasion test shows that the coating is not very strong with 1.6% transmittance decrease after 100 times rubbing. The transmittance increased to 99.93% when the calcination temperature increased to 280 °Ca little good for the optical property but very meaningful for the abrasion resistance. There was no transmittance decrease as behaved by the dense acid−SiO2 coating. Additional rubbing tests further certified the abrasion resistance of the F127−SiO2 coating, which also present in the two-layer coating system. To know more about the mechanical property of the coatings, nanoscratch and nanoindentation tests were carried out. The former described the variation of the friction coefficient with respect of the applied ramping load from 0 to 1500 μN. The friction refers to the force between the coating
Figure 7. Load−depth curves of acid−SiO2, F127−SiO2, and the double-layer coatings on glass substrates. The depths marked in the curves are corresponding to the applied load of 300 μN. The coating thicknesses are about 190, 220, and 420 nm, respectively, prepared based on a quarter of the wavelength of 1080 nm.
depth curves of the coatings tested with the same loading/ unloading conditions. It can be seen that for any given applied load there is an increase of the indentation depths from acid− SiO2 to F127−SiO2 to the double-layer coatings, which means a decreasing trend of their hardness. The hardness and reduced modulus were calculated using the famous Oliver−Pharr method38 put forward in 1992. The values are 4.0 and 68 10484
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(7) Pettit, R. B.; Brinker, C. J. Use of sol-gel thin films in solar energy applications. Sol. Energy Mater. 1986, 14, 269−287. (8) Bautista, M. C.; Morales, A. Silica antireflective films on glass produced by the sol-gel method. Sol. Energy Mater. Sol. Cells 2003, 80, 217−225. (9) Du, Y.; Luna, L. E.; Tan, W. S.; Rubner, M. F.; Cohen, R. E. Hollow silica nanoparticles in UV-visible antireflection coatings for poly(methyl methacrylate) substrates. ACS Nano 2010, 4, 4308−4316. (10) Moghal, J.; Kobler, J.; Sauer, J.; Best, J.; Gardener, M.; Watt, A. A.R.; Wakefield, G. High-performance, single-layer antireflective optical coatings comprising mesoporous silica nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 854−859. (11) Gemici, Z.; Shimomura, H.; Cohen, R. E.; Rubner, M. F. Hydrothermal treatment of nanoparticle thin films for enhanced mechanical durability. Langmuir 2008, 24, 2168−2177. (12) Moghal, J.; Reid, S.; Hagerty, L.; Gardener, M.; Wakefield, G. Development of single layer nanoparticle anti-reflection coating for polymer substrates. Thin Solid Films 2013, 534, 541−545. (13) Stöber, W.; Fink, A. Controlled growth of monodisperse silica spheres in the micron size range. J. Colloid Interface Sci. 1968, 26, 62− 69. (14) Xiao, Y. Q.; Shen, J.; Xie, Z. Y.; Zhou, B.; Wu, G. M. Microstructure control of nanoporous silica thin film prepared by solgel process. J. Mater. Sci. Technol. 2007, 23, 504−508. (15) Li, X. G.; Wang, X. D.; Shen, J. Broadband antireflective coating with different film designs by sol-gel method. Rare Metal Mater. Eng. 2012, 41, 144. (16) Li, X. G.; Shen, J. A scratch-resistant and hydrophobic broadband antireflective coating by sol-gel method. Thin Solid Films 2011, 519, 6236−6240. (17) Li, X.; Yu, X. H.; Han, Y. C. Intelligent reversible nanoporous antireflection film by solvent-stimuli-responsive phase transformation of amphiphilic block copolymer. Langmuir 2012, 28, 10584−10591. (18) Joo, W.; Park, M. S.; Kim, J. K. Block copolymer film with sponge-like nanoporous structure for antireflection coating. Langmuir 2006, 22, 7960−7963. (19) Shimizu, W.; Murakami, Y. Microporous silica thin films with low refractive indices and high Young’s modulus. ACS Appl. Mater. Interfaces 2010, 2, 3128−3133. (20) Park, M. S.; Kim, J. K. Broad-band antireflection coating at nearinfrared wavelengths by a breath figure. Langmuir 2005, 21, 11404− 11408. (21) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Silica aerogel films prepared at ambient pressure by using surface derivatization to induce reversible drying shrinkage. Nature 1995, 374, 439−443. (22) Zhao, D. Y.; Yang, P. D.; Melosh, N.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. Continuous mesoporous silica films with highly ordered large pore structures. Adv. Mater. 1998, 10, 1380−1385. (23) Li, W.; Yue, Q.; Deng, Y. H.; Zhao, D. Y. Ordered mesoporous materials based on interfacial assembly and engineering. Adv. Mater. 2013, 25, 5129−5152. (24) Zhao, D. Y.; Yang, P. D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Synthesis of continuous mesoporous silica thin films with three-dimensional accessible pore structures. Chem. Commun. 1998, 2499−2500. (25) Mitra, A.; Jana, D.; De, G. A facile synthesis of cubic (Im3m) alumina films on glass with potential catalytic activity. Chem. Commun. 2012, 48, 3333−3335. (26) Chhajed, S.; Schubert, M. F.; Kim, J. K.; Schubert, E. F. Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics. Appl. Phys. Lett. 2008, 93, 251108. (27) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle soot as a template for a transparent robust superamphiphobic coating. Science 2012, 335, 67−70. (28) Brinker, C. J.; Lu, Y. F.; Sellinger, A. Evaporation-induced selfassembly: Nanostructures made easy. Adv. Mater. 1999, 11, 579−585.
GPa for the acid−SiO2 coating, 3.2 and 68 GPa for the F127− SiO2 coating, 2.3 and 48 GPa for the double-layer coating. It should be noted that these values are higher than the true ones, especially the data of the single-layer coatings, due to the effect of the substrate. According to Oliver and Pharr,38 the insertion depth should be limited to less than 10% of the coating thickness to avoid the substrate effect. On the other hand, the depth should be more than 45 nm as a demand of the measurement equipment, which means that the tests of our coatings with the greatest thickness less than 450 nm were unavoidably affected by the glass substrates, and accordingly, the values of the double-layer coating are closest to the truth, so the gap between it and the other two coatings should be less than the tested values.
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CONCLUSIONS Dense and porous sol−gel SiO2 coatings with tough skeletons were prepared and incorporated into a double-layer broadband AR coating, which was less tough than the single layers but still very abrasion resistant. High transmittance over a wide wavelength range can be actualized with flexible preparation that technically allows the variation of refractive indices and relatively big operation error. The acid-catalyzed reaction system and the block copolymers F127 result in a dense chain skeleton and a mesoporous structure, respectively, which are responsible for the desirable mechanical property and optical performance. The method in this work may be suitable for some industrial production in view of the simplicity and low cost.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (J.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Grant No. U1230113), National Key Technology Research and Development Program of China (Grant No. 2013BAJ01B01), and Shanghai Committee of Science and Technology (Grant No. 11 nm0501600) for financial support.
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